1. Field of the Invention
Embodiments of the invention relate to an active matrix organic light emitting display, and more particularly to an organic light emitting display and a method of compensating for mobility thereof.
2. Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (hereinafter, abbreviated to “OLEDs”) capable of emitting light by itself and has advantages of a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, a light emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the light emitting layer EML and form excitons. As a result, the light emitting layer EML generates visible light.
The organic light emitting display arranges pixels each including the OLED in a matrix form and adjusts a luminance of the pixels depending on a gray scale of video data. Each pixel includes a driving thin film transistor (TFT) for controlling a driving current flowing in the OLED. It is preferable that electrical characteristics (including a threshold voltage, a mobility, etc.) of the driving TFT are equally designed in all of the pixels. However, in practice, the electrical characteristics of the driving TFTs of the pixels are not uniform due to various causes. A deviation between the electrical characteristics of the driving TFTs results in a luminance deviation between the pixels.
Various compensation methods of compensating for the deviation between the electrical characteristics of the driving TFTs are known. The compensation methods are classified into an internal compensation method and an external compensation method. The internal compensation method automatically compensates for a deviation between the threshold voltages of the driving TFTs inside circuits of the pixels. A driving current flowing in the OLED has to be determined irrespective of the threshold voltage of the driving TFT, so as to perform the internal compensation method. Therefore, configuration of the pixel circuit is very complex. Furthermore, the internal compensation method is not suitable to compensate for a deviation between mobilities of the driving TFTs.
The external compensation method measures sensing voltages corresponding to the threshold voltages (or mobilities) of the driving TFTs and modulates video data through an external circuit based on the sensing voltages, thereby compensating for a deviation between the threshold voltages (or mobilities). In the external compensation method, in general, after the deviation between the threshold voltages is compensated, the deviation between the mobilities is compensated. However, in recent, as a resolution of a display panel gradually increases, improving process capability and mass production, etc. are becoming issues. For these reasons, a simpler configuration of the pixel circuit is desired. Hence, the configuration of the pixel circuit applied to the external compensation method needs to be simpler.